RELATED PRIOR ART
In this application I shall use the word "soap", or if there is possible ambiguity, "ordinary soap", to describe the salt of an alkali metal or amine and a fatty carboxylic acid. Prior to the 1950's most agents added to water for the washing of laundry were based upon ordinary soap. In the 1950's major manufacturers throughout the world introduced detergent systems based upon synthetic detergents (syndets) for the washing of laundry.
Shortly after their introduction, such systems came to dominate the market for laundry detergents. There were a number of reasons for this rapid acceptance. At the time alkylbenzene sulfonic acid (ABS) had been discovered. This material is outstanding as a detergent, when properly formulated, and, at the time, was both inexpensive, and environmentally acceptable.
Later, in the 1960's, it was found that ABS is so persistent in the environment that it tended to cause streams and lakes to foam. This lack of biodegradability was identified as arising from the branched side alkyl chain of the ABS and was rectified (achievement of about 80% biodegradability) by using a liner alkyl side chain. Thus LAS, which stands for linear alkylbenzene sulfonic acid, replaced ABS. Detergents based upon this material dominate the world market at present.
Laundry detergent systems are not just a detergent and water, however, and involve considerable subtlety in their formulation. Because it is hard to motivate the present invention without some discussion of these formulations, such systems are briefly described.
LAS foams well and does not precipitate in hard water, however, it is not so effective a detergent as when used with soft water. The reason for this difference is that multivalent ions in the wash water adhere to and modify the surface charge of the clothes being washed so as to prevent proper lifting of soil from the fabric.
LAS in soft water is effective at removing soil, but is not so effective at preventing redeposition. As a result additives that prevent such redeposition, in particular, carboxymethylcelluloses, are routinely added to detergent formulations.
Many washing machines use aluminum components. Such components are chemically somewhat fragile, so that it is normal to add some sodium silicate to a laundry detergent system to prevent corrosion of the aluminum parts. The silicate, under the alkaline conditions of the wash water, forms an impervious coating of sodium aluminum silicate on the aluminum that prevents further attack. Currently plastic parts are being used more and more, however, so that, in time, it is anticipated that requirements for silicate will disappear.
Finally it is generally true that LAS when combined with about twenty percent of its own weight of a nonionic surfactant becomes especially effective for cleaning. Though it is possible to base a detergent system on either a nonionic or LAS, this particular combination is more effective than either alone and costs less since less detergent is required.
In summary, a detergent system ideally consists of a detergent combination, but mostly LAS, a water softener, usually a polyphosphate or zeolite in the United States, sodium silicate, carboxymethylcellulose, and a little perfume and perhaps some fluorescing dye (to dye the clothes whiter than white). Such systems may be supplemented by the inclusion of salts such as sodium sulfate to bulk out the detergent and to modify the ionic strength of the water in the wash. Such a modification is reputed to be helpful in the detergentcy of a system.
Very recently it has become fashionable to reduce the amount of material that must be added to the washing machine to do a load of laundry. As a result laundry detergent systems have had to dispense with any extra material and present day detergents may have reduced levels of all but the most key ingredients. With present liquid systems, some detergents have eliminated the water softening agents altogether, probably to the detriment of the cleaning ability of the system. Presently about 60 grams of detergent are used per medium load of wash in the United States.
Within the past two decades, it was found that by combining soaps with somewhat lesser amounts of certain detergents, which, when so used, are called lime soap dispersants, ordinary soap can be used in hard water. With most lime soap dispersants, the amount required depends on the hardness of the water and the amount of water used. In at least one case, that of certain sulfobetaines, the soap and lime soap dispersant need only be mixed in certain proportions and the combination is effective in almost any hardness at any dilution. This approach to using soaps is being used by some individuals at present, primarily for environmental reasons.
In spite of extensive work with detergent systems over the past years, some defects remain. Generally present day detergents leave clothes with a harsh handle, that is, hard and boardy and with a coarse feel, and with considerable static cling. As a result materials have been developed that may be added to the wash or dryer that soften clothes and eliminate static cling. Generally the materials that perform this function are certain fatty quaternary amines that surface treat fabrics and impart a softer feel. They also give better static discharge characteristics.
Over the years syndets have become relatively more expensive compared to soaps. The increase in cost of petroleum products compared to natural fats and oils, and the cost of the LAS as compared to ABS have caused this change. Syndets are only about 80% biodegradable and though such a level permits their use at present, future population growth may change the present picture in the United States. In some places, an example being Niagara Falls, present levels of stream contamination are giving problems with stream foaming.